Currently, ultrasound contrast agents enable the study of perfusion or distribution of blood supply within body tissues. Such contrast agents commonly comprise small microbubbles or gas-filled spheres and are strong scatterers of ultrasound signals. Hence, if they are injected into the blood supply of an organ or other tissue, their passage can be detected by examining the increase in back-scattered ultrasonic signals, using standard ultrasound imaging equipment.
It is known that some contrast agents provide a second harmonic response to impinging ultrasound energy at pressure amplitudes below a pressure which causes destruction thereof. Further, at moderately high pressure amplitudes, the response of microbubbles to ultrasound energy can be highly nonlinear, as a result of rupture of the microbubbles. This acoustically induced destruction and collapse of the microbubbles produces an even larger second nonlinear response to interrogating ultrasound signals.
U.S. Pat. No. 5,255,683 to Monaghan interrogates a tissue sample (in the absence of a contrast agent) during a first time period. During a second time period, after administration of the contrast agent, the same tissue sample is interrogated. After the first interrogation, the frequency characteristic data of the ultrasound energy reflected by the tissue is determined. During the second time period, frequency characteristic data of the reflected ultrasound energy is again acquired. The two sets of frequency data are then compared to obtain an indication of the presence of the agent in the tissue. Accordingly, the Monaghan system requires storage data pertaining to the frequency characteristics of the first scan and of the second scan, in addition to requiring substantial data processing to enable correlation of the first imaged tissue section to the second imaged tissue section. Such correlation is difficult to achieve and renders the Monaghan system impractical in application.
U.S. Pat. No. 5,456,257 to Johnson et al. describes an ultrasonic diagnostic system which detects the presence of contrast agents. Return echo energy is detected both before and after destruction of the contrast agent, during successive high-power scan lines. The return energies are subtracted to derive a measure of echo energy due to the destroyed contrast agent only. As with the Monaghan system described above, the Johnson et al. system requires plural scan lines to be acquired to enable the subtraction of data from both pre and post-destruction of the contrast agent.
U.S. Pat. No. 5,577,505 to Brock-Fisher et al., assigned the same Assignee as this application, describes an ultrasonic imaging system wherein non-linear responses of a contrast agent are measured, using multiple excitation levels. The responses gathered from the multiple excitation levels are gain-corrected in an amount corresponding to the difference in excitation levels and are then subtracted. Because of this subtraction, most of the linear response is removed and what remains corresponds to the non-linear response from the contrast agent. Here again, plural scan lines are required to gather the data identifying the return signals from the contrast agent.
Accordingly, there is a need for an improved ultrasound imaging system and method wherein contrast agent is identified using a single interrogation scan line. Further, the system should be able to detect the presence of contrast agent through use of lower power signal transmission levels so as to achieve longer contrast agent lifetime in the perfused tissue, and as result, lesser required dosages of the infused contrast agent.